Pressure sensor including electrical conductors comprising electroconductive resin composition that does not need cross-linking

ABSTRACT

A pressure sensor includes an insulator having a hollow portion; and a plurality of electrical conductors that have been disposed apart from each other along the inner surface facing the hollow portion of the insulator. The insulator comprises an insulating resin composition made of a material which does not need cross-linking, the plurality of electrical conductors comprise an electroconductive resin composition made of a material which does not need cross-linking, the insulating resin composition and the electroconductive resin composition comprise a process oil, and a mass percentage concentration of the process oil in the electroconductive resin composition is higher than a mass percentage concentration of the process oil in the insulating resin composition.

TECHNICAL FIELD

The present invention relates to an electroconductive resin compositionand a pressure sensor.

BACKGROUND ART

Conventional pressure sensors in which plural electrical conductors arearranged apart from each other are known (see, e.g., PTLs 1 and 2). Suchpressure sensors become in an electrically conducted state by contactbetween the plural electrical conductors when receiving an externalforce, thereby functioning as a switch.

The pressure sensor described in PTL 1 has a structure in which fourlinear electrode wires are spirally arranged with spaces therebetween soas not to be in contact with each other along an inner peripheralsurface of a cylindrical insulation, and is thereby capable of reliablydetecting an external force in all directions. These electrode wires areeach composed of an electrical conductor formed by twisting tin-platedsoft copper wires, and a conductive rubber covering the surface thereof.It is disclosed that the conductive rubber contains carbon black.

A cord switch described in PTL 2 is a linear pressure sensor and has astructure in which an end portion of a rubber cover covering the outerperiphery of a cylindrical insulation is sealed with a hot-melt resin.Curing time of a sealant is reduced since the hot-melt resin is used asthe sealant, and it is thus possible to efficiently manufacture the cordswitch. The plural electrical conductors provided in this cord switchare each composed of a core wire and a conductive resin covering thesurface thereof. The details of the conductive resin are not disclosed.

Meanwhile, a highly processable conductive rubber composition used as amaterial for electrical conductor of pressure sensor is known (see,e.g., PTL 3). The conductive rubber composition described in PTL 3 isobtained by adding an ethylene-α-olefin copolymer and carbon as aconductivity imparting agent to a base rubber having a Mooney viscosityML₁₊₄ (100° C.) of not more than 40.

CITATION LIST Patent Literature

[PTL 1]

-   JP-B-3275767

[PTL 2]

-   JP-A-2005-149760

[PTL 3]

-   JP-A-2011-162745

SUMMARY OF INVENTION Technical Problem

It is one of objects of the invention to provide an electroconductiveresin composition which can be produced at low cost and is suitable as amaterial for electrical conductors of a pressure sensor, and a pressuresensor including electrical conductors constituted of theelectroconductive resin composition.

Solution to Problem

To achieve the object described above, an embodiment of the inventionprovides a pressure sensor comprising: an insulator having a hollowportion; and a plurality of electrical conductors that have beendisposed apart from each other along the inner surface facing the hollowportion of the insulator, wherein the plurality of electrical conductorscomprise an electroconductive resin composition that includes both astyrene-based thermoplastic elastomer and carbon.

Also, another embodiment of the invention provides an electroconductiveresin composition, comprising: a styrene-based thermoplastic elastomerand carbon.

Advantageous Effects of Invention

According to the invention, it is possible to provide anelectroconductive resin composition which can be produced at low costand is suitable as a material for electrical conductors of a pressuresensor, and a pressure sensor including electrical conductorsconstituted of the electroconductive resin composition.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a radial cross sectional view showing a linear pressure sensorin an embodiment.

FIG. 2A is a schematic diagram illustrating the internal state of anelectrical conductor at a temperature higher than a melting point of acrystalline polyolefin.

FIG. 2B is a schematic diagram illustrating the internal state of theelectrical conductor at a temperature lower than a melting point of acrystalline polyolefin.

FIG. 3 is a graph showing a relation between a carbon concentration(mass %) and volume resistivity (Ohm·cm) in electroconductive resincompositions of Examples respectively when containing and when notcontaining a crystalline polyolefin.

DESCRIPTION OF EMBODIMENT

[Embodiment]

(Configuration of Pressure Sensor)

FIG. 1 is a radial cross sectional view showing a linear pressure sensor1 in the embodiment.

The pressure sensor 1 has an insulator 10 having a hollow portion 13 andplural electrical conductors 11 arranged apart from each other along aninner surface of the insulator 10 facing the hollow portion 13. Theelectrical conductor 11 covers a core wire 12, and the electricalconductor 11 and the core wire 12 constitutes an electrode wire 14 ofthe pressure sensor 1. Two electrical conductors 11 come into contactwith each other and are electrically conducted when an external force isapplied to the pressure sensor 1, and the pressure sensor 1 therebyfunctions as a switch.

The pressure sensor 1 is a linear pressure sensor, in which twoelectrode wires 14 are spirally arranged with a space so as not to be incontact with each other. However, the structure of the pressure sensor 1is not limited to such a structure. For example, the number of theelectrical conductors 11 may be three, and the insulator 10 and theelectrical conductor 11 may have a shape other than a linear shape, suchas a flat plate shape.

The core wire 12 is, e.g., a twisted wire composed of 26 to 30 AWGsilver-plated soft copper wires.

The electrical conductor 11 is formed of an electroconductive resincomposition containing a styrene-based thermoplastic elastomer andcarbon. It is not necessary to cross-link the styrene-basedthermoplastic elastomer when molding. Therefore, it is possible tosimplify the manufacturing process of the electrical conductor 11 and toreduce the manufacturing cost, as compared to when using a materialwhich needs to be cross-linked when molding, such as EPDM(ethylene-propylene-diene rubber).

The styrene-based thermoplastic elastomer is a thermoplastic elastomerwith styrene blocks at both ends of its molecules. Examples of thestyrene-based thermoplastic elastomer include SEBS with styrene blocksat both ends of EB (ethylene-butylene), SEPS with styrene blocks at bothends of EP (ethylene-propylene) and SEEPS with styrene blocks at bothends of EEP (ethylene-ethylene-propylene).

The molecular weight of the styrene-based thermoplastic elastomer ispreferably about 100,000 to 200,000. When the molecular weight is morethan 200,000, the surface of the electrical conductor 11 after extrusionmolding may be rough. It is considered that this is because, when themolecular weight of the styrene-based thermoplastic elastomer is toolarge, molecular chains are less likely to entangle at the time ofkneading and this decreases dispersibility of a crystalline polyolefin(described later), etc.

The electroconductive resin composition constituting the electricalconductor 11 preferably contains a crystalline polyolefin. When kneadinga material of the electrical conductor 11 in an extruder at atemperature higher than the melting point of the crystalline polyolefin,the material of the electrical conductor is kneaded in a state that thecrystalline polyolefin is in the form of liquid. Then, after molded intothe electrical conductor 11, the crystalline polyolefin crystallizes inthe electrical conductor 11 at ambient temperature (25° C.).

FIG. 2A is a schematic diagram illustrating the internal state of theelectrical conductor 11 at a temperature higher than a melting point ofthe crystalline polyolefin. In FIG. 2A, a region surrounded by a dottedline shows a liquid crystalline polyolefin 21 a in a styrene-basedthermoplastic elastomer 20. As shown in FIG. 2A, carbon 22 is containedin the styrene-based thermoplastic elastomer 20 as well as in the liquidcrystalline polyolefin 21 a at a temperature higher than the meltingpoint of the crystalline polyolefin.

FIG. 2B is a schematic diagram illustrating the internal state of theelectrical conductor 11 at a temperature lower than a melting point ofthe crystalline polyolefin. A crystallized crystalline polyolefin 21 bshown in FIG. 2B is a result from crystallization of the liquidcrystalline polyolefin 21 a caused by temperature drop. Due to thecrystallization of the crystalline polyolefin, the carbon 22 containedin the liquid crystalline polyolefin 21 a is pushed out into thestyrene-based thermoplastic elastomer 20. Then, many conductive pathsare formed by the resulting aggregation of carbon and conductivity ofthe electrical conductor 11 is improved. In addition, since conductivitycan be obtained with the smaller amount of the carbon 22 than when notusing the crystalline polyolefin, it is possible to reduce the amount ofthe carbon 22, thereby reducing the manufacturing cost of the electricalconductor 11.

Generally, kneading in an extruder is carried out at a temperature ofabout 180 to 230° C. Therefore, it is preferable to use a crystallinepolyolefin having a melting point of not more than 180° C. and also notless than 25° C. (ambient temperature). As such a crystallinepolyolefin, it is possible to use, e.g., polypropylene having a meltingpoint of about 140 to 160° C., polyethylene having a melting point ofabout 100 to 140° C., or EVA (ethylene-vinyl acetate copolymer) having amelting point of about 80° C. It is particularly preferable to usepolypropylene as the crystalline polyolefin since it shows affinity forthe styrene-based thermoplastic elastomer and the particle size aftercrystallization is not too large due to good dispersibility.

When using polypropylene as the crystalline polyolefin, a ratio of themass of the polypropylene contained in the electrical conductor 11 tothe mass of the carbon contained in the electrical conductor 11 ispreferably not less than 0.20 and not more than 1.10. It is possible toefficiently improve conductivity of the electrical conductor 11 when theratio is not less than 0.20. On the other hand, when the ratio is morethan more than 1.10, a low-temperature elastic modulus of the electricalconductor 11 exceeds the desired value.

In addition, the polypropylene is preferably of a reactor blend type.When using, e.g., a block copolymer polypropylene, the low-temperatureelastic modulus of the electrical conductor 11 tends to exceed thedesired value.

The carbon in the electroconductive resin composition constituting theelectrical conductor 11 is added for the purpose of impartingconductivity to the electrical conductor 11. The carbon is preferablyparticulate carbon such as carbon black. When using carbon in the formother than particle, e.g., in the form of wire or sheet, the level ofelectrical resistivity of the electrical conductor 11 varies dependingon a direction, which may adversely affect the operation of the pressuresensor 1. Here, carbon black is fine particles of carbon obtained by theincomplete combustion of oil or gas and having a diameter of about 3 to500 nm. Conductive carbon black having a developed structure ofconnected elementary carbon particles is particularly preferable.

In addition, it is preferable that the average particle size of theparticular carbon be smaller than the average particle size of thecrystalline polyolefin after crystallization. In this case, movement ofcarbon from liquid crystalline polyolefin into the styrene-basedthermoplastic elastomer caused by crystallization of the crystallinepolyolefin as shown in FIGS. 2A and 2B is likely to occur and it is thuspossible to effectively improve conductivity of the electrical conductor11. The average particle size of, e.g., polypropylene aftercrystallization is about 0.1 to 1.0 μm. As such, it is possible toeffectively improve conductivity of the electrical conductor 11 by usingcarbon black and polypropylene respectively as the carbon and thecrystalline polyolefin.

A mass percentage concentration of the carbon in the electroconductiveresin composition constituting the electrical conductor 11 is preferablynot less than 18 mass %. The electrical conductor 11 thereby have asufficiently small volume resistivity. The mass percentage concentrationof the carbon is calculated by dividing the mass of the carbon by thetotal mass of the electroconductive resin composition (the mass of theelectrical conductor 11) and then multiplying the obtained value by 100.

The insulating resin composition constituting the insulator 10preferably contains a styrene-based thermoplastic elastomer in the samemanner as the electroconductive resin composition constituting theelectrical conductor 11. It is not necessary to cross-link thestyrene-based thermoplastic elastomer when molding. Therefore, it ispossible to simplify the manufacturing process of the insulator 10 andto reduce the manufacturing cost.

The insulating resin composition constituting the insulator 10 and theelectroconductive resin composition constituting the electricalconductor 11 contain a process oil. Preferably, a mass percentageconcentration of the process oil in the electroconductive resincomposition constituting the electrical conductor 11 is higher than amass percentage concentration of the process oil in the insulating resincomposition constituting the insulator 10. The process oil here is anoil to be added to the insulator 10 and the electrical conductor 11 toincrease plasticity and to reduce hardness, and also functions as amedium for the carbon in the electrical conductor 11.

The process oil in a region with its high concentration moves to aregion with low concentration. Therefore, if the mass percentageconcentration of the process oil in the electroconductive resincomposition constituting the electrical conductor 11 is lower than themass percentage concentration of the process oil in the insulating resincomposition constituting the insulator 10, the process oil in theinsulator 10 moves into the electrical conductor 11. This increases theinterparticle distance of the carbon in the electrical conductor 11 anddecreases conductivity of the electrical conductor 11. However, such aproblem can be prevented by adjusting the mass percentage concentrationof the process oil in the electroconductive resin compositionconstituting the electrical conductor 11 to be higher than the masspercentage concentration of the process oil in the insulating resincomposition constituting the insulator 10.

The process oil is preferably a paraffin-based oil. Low-temperatureelastic moduli of the insulator 10 and the electrical conductor 11 tendto exceed the desired values more when using a paraffin-based oilcomposed of linear molecules than when using, e.g., a naphthene-basedoil or an aromatic oil composed of planar molecules. It is consideredthat this is because intermolecular interaction is lower and fluidity isless likely to be reduced even at a low temperature in the oil composedof linear molecules than in the oil composed of planar molecules.

(Method of Manufacturing the Pressure Sensor)

A method of manufacturing the pressure sensor 1 will be described belowas an example.

Firstly, the electrical conductors 11 are formed by extrusion moldingusing an extruder to cover the surfaces of the core wires 12, therebyforming two electrode wires 14. Here, the cross-linking process is notperformed since the electrical conductor 11 does not contain a materialwhich needs to be cross-linked.

Next, a first linear spacer for forming the center portion of the hollowportion 13 and second linear spacers for forming the peripheral portionof the hollow portion 13 are formed.

Next, four second spacers and the two electrode wires 14 are alternatelyarranged around the first spacer and are then twisted together.

Next, the insulator 10 is formed by extrusion molding to cover the foursecond spacers and the two electrode wires 14 which are twisted togetheraround the first spacer.

Next, the first spacer and the second spacers are pulled out, therebyforming the hollow portion 13. The pressure sensor 1 is formed throughthe steps described above.

The manufacturing process of the pressure sensor 1 in the presentembodiment is based on the manufacturing process of a pressure sensordisclosed in Japanese Patent No. 3275767, but the electrical conductor11, or the electrical conductor 11 and the insulator 10, is/are notcross-linked at the time of molding.

(Effects of the Embodiment)

In the embodiment, an electrical conductor is formed of anelectroconductive resin composition which can be produced at low costand is suitable as a material for electrical conductors of a pressuresensor, thereby allowing a pressure sensor to be obtained at low cost.

EXAMPLES

(Evaluation of the Electroconductive Resin Composition)

Sixteen types of electroconductive resin compositions (Samples 1 to 16)having different constituent components or proportions were formed andevaluated for volume resistivity (Ohm·cm), Mooney viscosity, Shore Ahardness, and elastic modulus (MPa) at low temperature (−30° C.). Then,electrode wires were formed by extruding Samples 1 to 16 of theelectroconductive resin compositions to cover core wires, and extrudedappearance (outer appearance of the electrode wire) was evaluated.

The Mooney viscosity was measured at 180° C. by a method in accordancewith JIS K 6300-1. The value after 7 minutes of preheating and 4 minutesof shearing (ML₇₊₄ (180° C.)) was defined as Mooney viscosity. Thevolume resistivity was measured on Samples 1 to 16 of theelectroconductive resin compositions in the form of sheet by a method inaccordance with JIS K 7194 (four terminal four-probe method). The ShoreA hardness was measured on Samples 1 to 16 of the electroconductiveresin compositions in the form of sheet by a method in accordance withASTM D2240. The low-temperature elastic modulus was measured at afrequency of 10 Hz at −30° C. by a method in accordance with JIS K7244-4.

The electrode wires used for the evaluation of the extruded appearancewere formed by covering core wires (twisted wires each composed 26 AWGsilver-plated soft copper wires) with electrical conductors formed ofSamples 1 to 16 of the electroconductive resin compositions so as tohave an outer diameter of 1.0 mm.

Tables 1 and 2 below show the constituent components of Samples 1 to 16of the electroconductive resin compositions and the evaluation resultsof Samples 1 to 16.

TABLE 1 Product name Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample6 Sample 7 Sample 8 Carbon Ketjenblack EC-600JD 25 25 25 25 25 25 25 25Crystalline polyolefin WELNEX RGF4VM 5 10 15 20 25 15 15 PM580X 15Process oil LUCANT HC-40 60 60 60 60 60 60 60 60 Styrene-based SEPTON4055 35 30 25 20 15 25 25 thermoplastic elastomer SEPTON 4099 25Antioxidant IRGANOX 1010 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Total mass (kg)125.1 125.1 125.1 125.1 125.1 128.1 125.1 125.1 Carbon concentration(mass %) 20.0 20.0 20.0 20.0 20.0 21.9 20.0 20.0 (Crystallinepolyolefin/Carbon) Mass ratio 0.20 0.40 0.60 0.80 1.00 0.54 0.60 0.60(Crystalline polyolefin + Carbon) 24.0 28.0 32.0 36.0 40.0 33.6 32.032.0 concentration (mass %) Process oil concentration (mass %) 48.0 48.048.0 48.0 48.0 46.8 48.0 48.0 Evaluation item Target value Volumeresistivity ≤1.2 0.794 0.753 0.712 0.671 0.625 0.412 0.693 0.715 (Ohm ·cm) Mooney viscosity ≤150 106 101.9 97 92 88 145 101 84 [ML₇₊₄ (180°C.)] Shore A hardness 84 84 90 94 98 98 96 93 Low-temperature elastic≤500 178 254 330 406 482 430 352 340 modulus (MPa) Extruded appearance Δ◯ ◯ ◯ ◯ ◯ ◯ Δ

TABLE 2 Product name Sample 9 Sample 10 Sample 11 Sample 12 Sample 13Sample 14 Sample 15 Sample 16 Carbon Ketjenblack 25 24 19 22 25 28 25 25EC-600JD Crystalline polyolefin WELNEX 20 20 0 0 0 0 30 5 RGF4VM Processoil LUCANT HC-40 52 60 60 60 60 60 50 PW-380 10 60 Styrene-basedthermoplastic SEPTON 4055 30 30 40 40 40 40 10 35 elastomer AntioxidantIRGANOX 1010 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Total mass (kg) 127.1 134.1119.1 122.1 125.1 128.1 125.1 125.1 Carbon concentration (mass %) 19.717.9 16.0 18.0 20.0 21.9 20.0 20.0 (Crystalline polyolefin/Carbon) Massratio 0.80 0.83 0.00 0.00 0.00 0.00 1.20 0.20 (Crystalline polyolefin +Carbon) concentration 35.4 32.8 16.0 18.0 20.0 21.9 44.0 24.0 (mass %)Process oil concentration (mass %) 40.9 44.7 50.4 49.1 48.0 46.8 48.048.0 Evaluation item Target value Volume resistivity (Ohm · cm) ≤1.20.721 0.956 4.95 2.6 1.26 0.833 0.603 0.735 Mooney viscosity ML₇₊₄ (180°C.) ≤150 88 85 71.3 91.4 129.8 175 85 94 Shore A hardness 95 94 66 71 7380 98 90 Low-temperature elastic modulus ≤500 418 410 56 71.8 102 136558 530 (MPa) Extruded appearance ◯ ◯ ◯ ◯ Δ X ◯ Δ

Masses (kg) of the carbon, the crystalline polyolefin, the process oiland the styrene-based thermoplastic elastomer contained in eachelectroconductive resin composition sample are shown in the upper rowsof Tables 1 and 2. Meanwhile, the total mass (kg) of a material, thecarbon concentration (mass %), the mass ratio of the carbon to thecrystalline polyolefin, the concentration (mass %) of the total of thecrystalline polyolefin and the carbon, and the process oil concentration(mass %) in each electroconductive resin composition sample are shown inthe middle rows of Tables 1 and 2.

Ketjenblack EC-600JD used as the carbon is carbon black (manufactured byKetjen Black International Company). WELNEX RGF4VM used as thecrystalline polyolefin is a reactor blend type polypropylene(manufactured by Japan Polypropylene Corporation) which has a density of0.89 g/cm³, a MFR (Melt Flow Rate) of 6.0 g/10 min and a bending elasticmodulus of 280 MPa. PM580X used as the crystalline polyolefin is a blockcopolymer polypropylene (manufactured by SunAllomer Ltd.) which has adensity of 0.9 g/cm³, a MFR of 5.0 g/10 min and a bending elasticmodulus of 1300 MPa. LUCANT HC-40 used as the process oil is anethylene-α-olefin co-oligomer (manufactured by Mitsui Chemicals, Inc.)which has a kinetic viscosity of 40 mm²/s at 100° C. PW-380 used as theprocess oil is a mineral oil (manufactured by Idemitsu Kosan Co., Ltd.)which contains 27% of naphthene component and 73% of paraffin componentand has a kinetic viscosity of 30 mm²/s. SEPTON 4055 used as thestyrene-based thermoplastic elastomer is SEEPS (manufactured by KurarayCo., Ltd) which contains 30% of styrene component and has a molecularweight of 130,000. SEPTON 4099 used as the styrene-based thermoplasticelastomer is SEEPS (manufactured by Kuraray Co., Ltd) which contains 30%of styrene component and has a molecular weight of 320,000. IRGANOX 1010used as antioxidant is pentaerythritoltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (manufacturedby BASF) which is added to eliminate an effect of oxidative degradationin a heat-aging test (described later) conducted on the pressure sensor.

The target value for each evaluation is a value as a material for theelectrical conductor of the electrode wire of pressure sensor. Thesymbol “◯” in the section of Extruded appearance means that roughnesswas virtually not observed on the outer appearance, the symbol “Δ” meansthat slight roughness was observed, and the symbol “X” means thatroughness was observed and was to the extent that it is problematic forpractical use.

Samples 1 to 10 are the samples exhibiting satisfactory results for theevaluations of volume resistivity, Mooney viscosity, low-temperatureelastic modulus and extruded appearance. On the other hand, Samples 11to 16 are the samples exhibiting an unsatisfactory result for at leastone of the evaluations of volume resistivity, Mooney viscosity,low-temperature elastic modulus and extruded appearance.

When comparing Samples 3 and 7 which contain different types ofcrystalline polyolefins, Sample 3 had the better low-temperature elasticmodulus. It is considered that this is because Sample 3 contains areactor blend type polypropylene while Sample 7 contains a blockcopolymer polypropylene.

When comparing Samples 3 and 8 which contain different types ofstyrene-based thermoplastic elastomers, the electrode wire having anelectrical conductor formed of Sample 3 had the better result inextruded appearance. It is considered that this is becausedispersibility of the crystalline polyolefin was reduced in Sample 8since the molecular weight of the styrene-based thermoplastic elastomerwas too large.

Samples 11 and 14 did not contain a crystalline polyolefin and thus hada low volume resistivity. Table 2 shows that the volume resistivitydecreases with an increase in the carbon concentration in Samples 11 to14. However, even Sample 14 having the highest carbon concentrationthereamong has a higher volume resistivity than Samples 1 and 9.

FIG. 3 is a graph showing a relation between a carbon concentration(mass %) and volume resistivity (Ohm·cm) respectively when theelectroconductive resin composition contains and does not contain acrystalline polyolefin. The plot marks “♦” are the values of Samples 3,6, 9 and 10 and show a relation when the electroconductive resincomposition contains a crystalline polyolefin. The plot marks “●” arethe values of Samples 11 to 14 and show a relation when theelectroconductive resin composition does not contain a crystallinepolyolefin.

Meanwhile, the evaluation results of Samples 11 to 14 show that theMooney viscosity increases with an increase in the carbon concentrationin the electroconductive resin composition, which causes a decrease inmoldability and resulting deterioration in extruded appearance. On theother hand, for example, Sample 6 is excellent in extruded appearanceeven though the Mooney viscosity is high. It is considered that this isbecause moldability is less likely to decrease in the electroconductiveresin composition containing the crystalline polyolefin even if thecarbon concentration is increased.

Sample 15, which contains a crystalline polyolefin, achieved the targetvolume resistivity but had a low-temperature elastic modulus exceedingthe target value. It is considered that this is because a ratio of themass of the crystalline polyolefin to the mass of the carbon in Sample15 was too large. Based on the results that the ratio of the mass of thepolypropylene to the mass of the carbon was 1.20 in Sample 15 and theratio of the mass of the polypropylene to the mass of the carbon was notless than 0.20 and not more than 1.0 in Samples 1 to 8 which achievedthe target low-temperature elastic modulus, the preferred ratio of themass of the polypropylene to the mass of the carbon in theelectroconductive resin composition is considered to be about not lessthan 0.20 and not more than 1.0.

In Sample 16, a crystalline polyolefin was contained and the ratio ofthe mass of the crystalline polyolefin to the mass of the carbon was0.20 which is the same value as Sample 1, but the low-temperatureelastic modulus was more than the target value. It is considered thatthis is because the process oil in Sample 16 contains a naphthenecomponent.

(Evaluation of the Pressure Sensor)

Next, eight types of pressure sensors (Samples A to H) were formed sothat the respective electrical conductors of the electrode wires wereformed of electroconductive resin compositions composed of differentcomponents and the insulators were formed of insulating resincompositions composed of different components. Then, the ON resistance(Ohm), the ON resistance (Ohm) after the heat-aging test at 100° C. for1000 hours, the ON load (N), and the ON load (N) at low temperature(−30° C.) were evaluated.

The pressure sensor samples A to H here have an insulator having ahollow portion and two electrode wires formed in a double spiralstructure so as to be provided apart from each other along the innersurface in the hollow portion, in the same manner as the pressure sensor1 shown in FIG. 1. The shape of the hollow portion is also the same asthat of the pressure sensor 1 shown in FIG. 1. The electrode wires wereformed using Samples 3, 5, 6, 12 and 16. A space between the electrodewires was 1.8 mm and an outer diameter of the insulator was 5.0 mm.

In the ON resistance test, 20 N of load was applied to round bar-shapedindenters of 4 mm in diameter placed on the pressure sensor samples A toH so as to cross substantially orthogonal thereto, electrical resistanceat the moment of occurrence of electrical conduction between the twoelectrode wires was measured 10 times, and the average value thereof wascalculated. This average value is the ON resistance value in Table 4. Inthe ON resistance test after the heat-aging test, the same ON test wasconducted after the heat-aging test at 100° C. for 1000 hours.

In the ON load test, a load was gradually applied to round bar-shapedindenters of 4 mm in diameter placed on the pressure sensor samples A toH so as to cross substantially orthogonal thereto, and a load at whichelectrical resistance of the pressure sensor reached 100 Ohm wasmeasured. In the ON load test under the low temperature conditions, thesame ON load test was conducted at a temperature of −30° C.

Table 3 below shows three types of insulating resin compositions andprocess oil concentrations used for the insulators in Samples A to H.

TABLE 3 Formu- Formu- Formu- lation lation lation Product name L M NProcess oil LUCANT HC-40 100 82 85 Styrene-based SEPTON 4055 65 60 60thermoplastic elastomer Kraton RP6935 25 20 20 Heavy calcium carbonateSOFTON 1200 10 20 20 Antioxidant IRGANOX 1010 0.1 0.1 0.1 Process oilconcentration (mass %) 50.0 45.1 45.9

Kraton RP6935 used as the styrene-based thermoplastic elastomer is SEBS(manufactured by Kraton Polymers) which contains 58% of styrenecomponent and has a molecular weight of 200,000. SOFTON 1200 used asheavy calcium carbonate is heavy calcium carbonate (manufactured byBihoku Funka Kogyo Co., Ltd.) which has a particle size of 1.8 μm andhas an oil absorption of 36 cc/100 g.

Table 4 below shows the types of electroconductive resin compositionsused to form electrical conductors and insulating resin compositionsused to form insulators in Samples A to H, and the respective evaluationresults of Samples A to H.

TABLE 4 Sample A Sample B Sample C Sample D Electroconductive resinSample 3 Sample 3 Sample 5 Sample 6 composition Process oil 48.0 48.048.0 46.9 concentration (mass %) Insulating resin compositionFormulation M Formulation N Formulation M Formulation N Process oilconcentration (mass %) 45.1 45.9 45.1 45.9 Target Evaluation item valueON resistance (Ohm) ≤30 15.2 15.7 17.2 18.4 ON resistance after 16.416.3 18.1 19.0 Heat-aging test ON load (N) ≤15 3.6 3.4 3.8 4.1 ON load(N) under low 10.2 10.8 14.1 13.6 temperature conditions Sample E SampleF Sample G Sample H Electroconductive resin Sample 3 Sample 6 Sample 12Sample 16 composition Process oil 48.0 46.8 49.2 48.0 concentration(mass %) Insulating resin composition Formulation L Formulation LFormulation M Formulation L Process oil concentration (mass %) 50.0 50.045.1 50.0 Target Evaluation item value ON resistance (Ohm) ≤30 15.5 19.141.9 18.7 ON resistance after 52.3 42.8 44.7 54.4 Heat-aging test ONload (N) ≤15 4.4 3.7 4.2 4.0 ON load (N) under low 11.7 12.4 20.3 17.6temperature conditions

Samples A to D are the samples exhibiting satisfactory results for allfour types of evaluations. On the other hand, Samples E to D are thesamples exhibiting an unsatisfactory result for at least one of theevaluations.

The ON resistance values of Samples E, F and H after heat-aging weremore than the target value. It is considered that this is because theprocess oil concentration in Samples E, F and H was higher in theinsulating resin composition than in the electroconductive resincomposition, and the process oil thus moved from the insulator into theelectrical conductors.

Sample G exhibited the unsatisfactory results for both the ON resistanceand the post-heat-aging ON resistance since the electroconductive resincomposition constituting the electrical conductor was Sample 12 having alarge volume resistivity.

Summary Of The Embodiment

Technical ideas understood from the embodiment will be described belowciting the reference numerals, etc., used for the embodiment. However,each reference numeral, etc., described below is not intended to limitthe constituent elements in the claims to the members, etc.,specifically described in the embodiment.

[1] A pressure sensor (1), comprising: an insulator (10) having a hollowportion (13); and a plurality of electrical conductors (11) that havebeen disposed apart from each other along the inner surface facing thehollow portion (13) of the insulator (10), wherein the plurality ofelectrical conductors (11) comprise an electroconductive resincomposition that includes both a styrene-based thermoplastic elastomerand carbon.

[2] The pressure sensor (1) defined by [1], wherein theelectroconductive resin composition comprises a crystalline polyolefin.

[3] The pressure sensor (1) defined by [2], wherein the crystallinepolyolefin has a melting point of not less than 25° C. and not more than180° C.

[4] The pressure sensor (1) defined by [3], wherein the crystallinepolyolefin is polypropylene.

[5] The pressure sensor (1) defined by [4], wherein a ratio of the massof the polypropylene contained in the electroconductive resincomposition to the mass of the carbon contained in the plurality ofelectrical conductors (11) is not less than 0.20 and not more than 1.10.

[6] The pressure sensor defined by any one of [1] to [5], wherein theelectroconductive resin composition has a volume resistivity of not morethan 1.0 Ohm·cm.

[7] The pressure sensor (1) defined by any one of [1] to [5], wherein amass percentage concentration of the carbon in the electroconductiveresin composition is not less than 18 mass %.

[8] The pressure sensor (1) defined by any one of [1] to [5], whereinthe insulator (10) comprises an insulating resin composition containinga styrene-based thermoplastic elastomer.

[9] The pressure sensor (1) defined by [8], wherein the insulating resincomposition and the electroconductive resin composition comprise aprocess oil, and a mass percentage concentration of the process oil inthe electroconductive resin composition is higher than a mass percentageconcentration of the process oil in the insulating resin composition.

[10] The pressure sensor (1) defined by [9], wherein the process oil isa paraffin-based oil.

[11] The pressure sensor (1) defined by any one of [1] to [5], whereinthe carbon is in the form of particle.

[12] The pressure sensor (1) defined in any one of [2] to [5], whereinthe average particle size of the carbon is smaller than the averageparticle size of the crystalline polyolefin.

[13] The pressure sensor (1) defined by any one of [1] to [5], whereinthe insulator (10) has a cylindrical shape, and the plurality ofelectrical conductors (11) extend spirally in a longitudinal directionof the insulator (10) along the inner surface of the insulator (10).

[14] An electroconductive resin composition, comprising a styrene-basedthermoplastic elastomer; carbon; and a crystalline polyolefin.

[15] The electroconductive resin composition defined by [14], whereinthe crystalline polyolefin has a melting point of not less than 25° C.and not more than 180° C.

[16] The electroconductive resin composition defined by [15], whereinthe crystalline polyolefin is polypropylene.

[17] The electroconductive resin composition defined by [16], wherein aratio of the mass of the polypropylene to the mass of the carbon is notless than 0.20 and not more than 1.10.

[18] The electroconductive resin composition defined by any one of [14]to [17], wherein a volume resistivity is not more than 1.0 Ohm·cm.

[19] The electroconductive resin composition defined by any one of [14]to [17], wherein a mass percentage concentration of the carbon is notless than 18 mass %.

[20] The electroconductive resin composition defined by any one of [14]to [17], comprising a paraffin-based oil.

[21] The electroconductive resin composition defined by any one of [14]to [17], wherein the carbon is in the form of particle.

[22] The electroconductive resin composition defined by any one of [14]to [17], wherein the average particle size of the carbon is smaller thanthe average particle size of the crystalline polyolefin.

Although the embodiment and Examples of the invention have beendescribed, the invention is not to be limited to the embodiment andExamples, and the various kinds of modifications can be implementedwithout departing from the gist of the invention. For example, theinvention is characterized in the constituent components of theelectroconductive resin composition constituting the electricalconductor and is thus applicable to pressure sensors having anyconfigurations.

In addition, an antioxidant, an anti-aging agent, an antiozonant, ametal chelator, a flame retardant, a colorant, a foaming agent, alubricant, a stabilizer, a filler, a compatibilizing agent and areinforcing agent can be appropriately added to the resin compositionused for the electrical conductor or the insulator without departing thescope of the requirements of [1] to [20].

In addition, the invention according to claims is not to be limited tothe above-mentioned embodiment and Examples. Further, please note thatall combinations of the features described in the embodiment andExamples are not necessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided are an electroconductive resin composition which can beproduced at low cost and is suitable as a material for electricalconductors of a pressure sensor, and a pressure sensor includingelectrical conductors constituted of the electroconductive resincomposition.

REFERENCE SIGNS LIST

-   1: PRESSURE SENSOR-   10: INSULATOR-   11: ELECTRICAL CONDUCTOR-   12: CORE WIRE-   13: HOLLOW PORTION-   14: ELECTRODE WIRE-   20: STYRENE-BASED THERMOPLASTIC ELASTOMER-   21A, 21B: CRYSTALLINE POLYOLEFIN-   22: CARBON

The invention claimed is:
 1. A pressure sensor, comprising: an insulatorhaving a hollow portion; and a plurality of electrical conductors thathave been disposed apart from each other along the inner surface facingthe hollow portion of the insulator, wherein the insulator comprises aninsulating resin composition made of a material which does not needcross-linking, wherein the plurality of electrical conductors comprisean electroconductive resin composition made of a material which does notneed cross-linking, wherein the insulating resin composition and theelectroconductive resin composition comprise a process oil, and a masspercentage concentration of the process oil in the electroconductiveresin composition is higher than a mass percentage concentration of theprocess oil in the insulating resin composition.
 2. The pressure sensoraccording to claim 1, wherein the electroconductive resin compositionthat includes both a styrene-based thermoplastic elastomer and carbon.3. The pressure sensor according to claim 2, wherein theelectroconductive resin composition comprises a crystalline polyolefin.4. The pressure sensor according to claim 3, wherein the crystallinepolyolefin comprises a polypropylene.
 5. The pressure sensor accordingto claim 4, wherein a ratio of the mass of the polypropylene containedin the electroconductive resin composition to the mass of the carboncontained in the plurality of electrical conductors is not less than0.20 and not more than 1.10.
 6. The pressure sensor according to claim4, wherein the polypropylene comprises a reactor blend typepolypropylene.
 7. The pressure sensor according to claim 3, wherein thecarbon is in the form of a particle and the average particle size of thecarbon is smaller than the average particle size of the crystallinepolyolefin.
 8. The pressure sensor according to claim 2, wherein a masspercentage concentration of the carbon in the electroconductive resincomposition is not less than 18 mass %.
 9. The pressure sensor accordingto claim 2, wherein the carbon is in the form of a particle.
 10. Thepressure sensor according to claim 1, wherein the electroconductiveresin composition has a volume resistivity of not more than 1.0 Ohm·cm.11. The pressure sensor according to claim 1, wherein the process oilcomprises a paraffin-based oil.
 12. The pressure sensor according toclaim 11, wherein naphthene component is not contained in the processoil.
 13. The pressure sensor according to claim 11, wherein theelectroconductive resin composition comprises an elastic modulus of 500MPa or less at a temperature of −30° C.
 14. The pressure sensoraccording to claim 1, wherein the insulator has a cylindrical shape, andthe plurality of electrical conductors extend spirally in a longitudinaldirection of the insulator along the inner surface of the insulator.